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add a special projection for EPSG:27572 #2427

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merged 9 commits into from
Oct 7, 2024
Merged

add a special projection for EPSG:27572 #2427

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1a48244
add a projection for L2E
ThibHlln Aug 27, 2024
aafe3f5
add unit tests for L2E projection
ThibHlln Aug 27, 2024
20f1a6a
fix swapped lat/lon in unit test
ThibHlln Aug 27, 2024
5d38ab3
rename class to LambertZoneII
ThibHlln Sep 28, 2024
6c61a5c
update expected values in test as this is due to using PlateCarree in…
ThibHlln Sep 28, 2024
950b220
add test image for LambertZoneII
ThibHlln Sep 28, 2024
bb49798
fix CRS used to determine bounds
ThibHlln Sep 30, 2024
02d8842
update docstring to mention its geographical relevance
ThibHlln Sep 30, 2024
871222c
rely on EPSG code but redefine bounds manually
ThibHlln Sep 30, 2024
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54 changes: 54 additions & 0 deletions lib/cartopy/crs.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1815,6 +1815,60 @@ def y_limits(self):
return self._y_limits


class L2E(Projection):
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Should this inherit from LambertConformal if it is that type of projection? I think the naming should contain Lambert at least, here is a quick suggestion.

Suggested change
class L2E(Projection):
class LambertZoneII(LambertConformal):

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I have not inherited from LambertConformal because it does not make it possible to set some PROJ4 parameters, such as the scaling factor k_0.

"""
Lambert zone II projection (also known as "Lambert 2 étendue" in French)
(https://epsg.io/27572).

"""
def __init__(self):
proj4_params = [('proj', 'lcc'),
('lat_1', 46.8),
('lat_0', 46.8),
('lon_0', 0.0),
('k_0', 0.99987742),
('x_0', 600000.0),
('y_0', 2200000.0),
('pm', 'paris'),
('units', 'm'),
('no_defs', None)]

globe = Globe(semimajor_axis='6378249.2',
semiminor_axis='6356515.0',
towgs84='-168,-60,320,0,0,0,0')
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Where do these values come from? (I didn't see anything about this in the EPSG website) Is this projection only valid with this Globe and do we need to allow users to override this default value?

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This ellipsoid corresponds to EPSG:7011. These values (and the conversion to WGS84) can be found in a report [in French] produced by the French National Institute of Geographic and Forest Information in sections 3.1.1 and 4.3.1, respectively. As EPSG:27572 clearly defines the ellipsoid to use, I don't think we need to allow users to override them.


super().__init__(proj4_params, globe=globe)

x0, x1, y0, y1 = [0, 1.2e6, 1.6e6, 2.7e6]
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Same question about these bounds values

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Given the purpose of this projection is to cover hexagonal France and Corsica, these values include those two objects. They do not come from an official standard though. How else could they be chosen? I would gladly welcome suggestions on this and amend accordingly. :)

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I had a quick look at the EPSG version in your original post, and realized that the boundary there is being set by the area_of_use, which should be a good quantity we can reference for our boundary. The interesting thing here is that I think the ellipsoid/globe being used may not correspond to the area of use coordinates being reported.

Here is we are calling self.as_geodetic()

cartopy/lib/cartopy/crs.py

Line 688 in 694efcf

points = self.transform_points(self.as_geodetic(), lons, lats)

but here it says that it is values reported using the WGS84 ellipse.

cartopy/lib/cartopy/crs.py

Line 678 in 694efcf

# Geographic area of the entire dataset referenced to WGS 84

So, I think this may be a bug in the epsg code path and we should be doing something like:
points = self.transform_points(PlateCarree().as_geodetic(), lons, lats)

Thoughts on how that looks?

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Replacing with PlateCarree does fix the initial problem mentioned in the opening message of this PR. However, the resulting bounds are not wide enough to include the southmost part of hexagonal France and Corsica.

import matplotlib.pyplot as plt
import cartopy

ax = plt.subplot(projection=cartopy.crs.epsg(27572))
ax.set_extent(cartopy.crs.epsg(27572).bounds, crs=cartopy.crs.epsg(27572))
ax.add_feature(cartopy.feature.BORDERS)
ax.add_feature(cartopy.feature.COASTLINE)
ax.scatter(564055, 2571176, transform=cartopy.crs.epsg(27572))
plt.show()

myplot

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👍 My proposal would be to do something like this then to update the bounds manually after initialization from the EPSG crs.

class LambertZoneII(Projection):
    def __init__(self):
        crs = pyproj.CRS.from_epsg(27572)
        super().__init__(crs.to_wkt())
        # Projected bounds from https://epsg.io/27572
        self.bounds = [-5242.32, 1212512.16, 1589155.51, 2706796.21]

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Ok, I've done just that. Works just fine. :-)
I have updated the test image too, as the extent is now slightly different from the one I had arbitrarily set before.
Thanks.


self.bounds = (x0, x1, y0, y1)

self._boundary = sgeom.LineString(
[(x0, y0), (x0, y1), (x1, y1), (x1, y0), (x0, y0)]
)

self._x_limits = [x0, x1]
self._y_limits = [y0, y1]

self._threshold = min(x1 - x0, y1 - y0) / 100.

@property
def boundary(self):
return self._boundary

@property
def x_limits(self):
return self._x_limits

@property
def y_limits(self):
return self._y_limits

@property
def threshold(self):
return self._threshold


class LambertAzimuthalEqualArea(Projection):
"""
A Lambert Azimuthal Equal-Area projection.
Expand Down
26 changes: 26 additions & 0 deletions lib/cartopy/tests/crs/test_lambert_conformal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -114,3 +114,29 @@ def test_single_npole(self):
assert_array_almost_equal(n_pole_crs.y_limits,
expected_y,
decimal=0)


class TestL2E:
def setup_class(self):
self.point_a = (1.4868268900254693, 48.13277955695077)
self.point_b = (-2.3188020040300126, 48.68412929316207)
self.src_crs = ccrs.PlateCarree()
self.nan = float('nan')

def test_default(self):
proj = ccrs.L2E()
res = proj.transform_point(*self.point_a, src_crs=self.src_crs)
np.testing.assert_array_almost_equal(res,
(536745.89626, 2348522.77899),
decimal=5)
res = proj.transform_point(*self.point_b, src_crs=self.src_crs)
np.testing.assert_array_almost_equal(res,
(257266.90019, 2419663.00145),
decimal=5)

def test_nan(self):
proj = ccrs.L2E()
res = proj.transform_point(0.0, float('nan'), src_crs=self.src_crs)
assert np.all(np.isnan(res))
res = proj.transform_point(float('nan'), 0.0, src_crs=self.src_crs)
assert np.all(np.isnan(res))
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